Abstract

Mycobacterium microti is a member of the Mycobacterium tuberculosis complex of bacteria. This species was
originally identiﬁed as a pathogen of small rodents and shrews and was associated with limited diversity and
a much reduced spoligotype pattern. More recently, speciﬁc deletions of chromosomal DNA have been shown
to deﬁne this group of organisms, which can be identiﬁed by the absence of chromosomal region RD1mic. We
describe here the molecular characteristics of 141 strains of the Mycobacterium tuberculosis complex isolated in
Great Britain over a 14-year period. All strains have characteristic loss of some spoligotype spacers and
characteristic alleles at the ETR-E and ETR-F variable-number tandem-repeat (VNTR) loci, and a sample of
these strains was deleted for regions RD7, RD9, and RD1micbut intact for regions RD4 and RD12. We therefore
identiﬁed these strains as M. microti and show that they have much more diverse spoligotype patterns and
VNTR types than previously thought. The most common source of these strains was domestic cats, and we show
that the molecular types of M. microti are geographically localized in the same way that molecular types of
Mycobacterium bovis are geographically localized in cattle in the United Kingdom. We describe the pathology
of M. microti infection in cats and suggest that the feline disease is a spillover from a disease maintained in an
unknown wild mammal, probably ﬁeld voles. The location of the cats with M. microti infection suggests that
they do not overlap geographically with the strains of Mycobacterium bovis in Great Britain